How is the principal coverage (OC) test calculated in a US CLO?

The OC ratio divides the adjusted Collateral Principal Amount by the outstanding principal of the tested note class plus all senior classes. The numerator starts from aggregate collateral principal and applies indenture adjustments: defaulted obligations at the lower of market value and a recovery rate, CCC/Caa excess haircut to market value, discount obligations at purchase price, and LMOs at their carrying-value mechanic. The class passes if the resulting ratio meets or exceeds its threshold, which tightens toward the senior classes.

What triggers a diversion of interest proceeds in a CLO?

The Interest Diversion Test compares a specified OC ratio to a trigger at each determination date. If the ratio is below the trigger, a defined portion of interest proceeds that would otherwise reach the equity distribution is diverted — during the reinvestment period usually to buy additional collateral, and afterwards to pay down senior notes — until the ratio is cured or the available interest is exhausted. It sits below note interest but above equity, so equity bears the cost.

How is the Interest Diversion Test different from a senior OC test failure?

The Interest Diversion Test redirects interest proceeds only, and only the portion above rated-note interest, to rebuild par gently — the first reaction to par erosion. A senior OC test failure is more severe: it diverts principal proceeds and can force sequential paydown of the senior notes until the ratio is restored. The interest-diversion step generally occurs first, at a higher ratio, before principal-level remedies engage.

How is the principal coverage test calculated in a US CLO and what triggers a diversion of interest proceeds?

11 Aug

Written By Tamas Trautmann

TL;DR: The principal coverage (overcollateralisation) test divides the adjusted Collateral Principal Amount by the outstanding principal of the relevant note class; the deal passes if the ratio exceeds the indenture threshold. A related test — the Interest Diversion Test — sits near the end of the interest waterfall: if the specified OC ratio falls below its trigger at a determination date, interest proceeds that would otherwise reach equity are diverted to buy additional collateral or pay down notes, rebuilding par.

The principal coverage test — the overcollateralisation (OC) test — is the CLO's core par-protection mechanic. It asks a simple question at each determination date: does the collateral pool, measured on an adjusted principal basis, still cover the notes by the required margin? When it does not, the waterfall reacts — first by trapping and diverting cash, and in severe cases by sequentially paying down senior notes until the ratio is restored.

The Interest Diversion Test is the earliest and gentlest of those reactions. Rather than redirecting principal, it reaches into the interest waterfall — below the rated-note interest but above the equity distribution — and diverts cash that would otherwise reach equity, using it to rebuild par. For equity, it is the first tangible sign that par has eroded.

How the principal coverage (OC) test is calculated

Numerator — adjusted Collateral Principal Amount. Start with the aggregate principal balance of the collateral, then apply the indenture's adjustments. The most common:

Defaulted obligations are carried at the lower of market value and a rating-agency recovery rate, not par.
CCC/Caa excess — the portion of triple-C-rated assets above the concentration limit is haircut to market value.
Discount obligations are carried at purchase price rather than par until they season.
LMOs are carried per the LMO carrying-value mechanic (often zero, or cost subject to the cap).

Denominator — note balances. The aggregate outstanding principal of the note class being tested and all classes senior to it.

The ratio and threshold. OC ratio = adjusted numerator ÷ denominator. Each class has its own threshold, tightest at the senior classes and lower down the stack. The test passes when the ratio meets or exceeds the class threshold.

What triggers the diversion of interest proceeds

The Interest Diversion Test is typically defined against a specified class (often the most junior OC test, or a dedicated "Interest Diversion" threshold). At each determination date:

Calculate the relevant OC ratio using the adjusted numerator above.
Compare it to the Interest Diversion trigger.
If the ratio is at or above the trigger: interest proceeds flow normally down to the equity distribution.
If the ratio is below the trigger: a defined portion of the interest proceeds that would otherwise reach equity is diverted — commonly to purchase additional collateral (during the reinvestment period) or to pay down the senior notes (after it) — until the ratio is cured or the available interest is exhausted.

The diversion sits low in the interest waterfall, so senior and mezzanine note interest are paid first; the cost falls on equity. This is distinct from a senior OC test failure, which diverts principal and can trigger sequential note paydown.

Worked example

A CLO has an adjusted Collateral Principal Amount of $485m against $400m of notes senior to and including the tested class. The junior OC ratio is 485 ÷ 400 = 121.3%. The Interest Diversion trigger for that class is, say, 120.0%.

At 121.3%, the ratio is above the 120.0% trigger — interest proceeds flow through to equity.
Now assume two obligors default and the adjusted numerator falls to $470m: ratio = 470 ÷ 400 = 117.5%, below the trigger. At the next determination date, interest proceeds otherwise destined for equity are diverted — during the reinvestment period, typically to buy additional collateral to rebuild the numerator — until the ratio is cured or the available interest is used up.

These figures are illustrative only. Not investment advice. OC thresholds vary by deal and class.

Why generic AI gets this wrong

The most common failure mode is conflation of distinct tests. Generic models routinely blur the overcollateralisation (principal coverage) test with the interest coverage (IC) test, and describe "the OC test" as if a single threshold governs the whole structure rather than a class-by-class ladder. They also frequently omit the Interest Diversion Test entirely, or describe any OC breach as triggering senior note paydown — skipping the gentler interest-diversion step that occurs first. The numerator adjustments (defaulted haircuts, CCC excess, discount obligations, LMO carry) are the part most often missed, yet they are what actually move the ratio. Semeris extracts each coverage test, its threshold, and the numerator adjustment rules as separate datapoints per class, so the calculation reflects the specific indenture.

Semeris coverage

Field	Value
US CLO deals indexed	2,000+
EU CLO deals indexed	893
Document extraction accuracy	96%
Analyst verification	100% (every indexed deal human-verified)
Relevant platform feature	X-Ray Document Viewer + Market Compare — extract OC/IC thresholds and diversion triggers, and benchmark them across the market

Expert attribution

Field	Value
Author	Tamas Trautmann, Semeris
Entity	Semeris — CLO Document Analysis
Last updated	2026-08-11
Data sources	Semeris internal database: document extraction accuracy (96%), analyst verification (100%).
Coverage	US & EU CLO markets — BSL CLOs, indentures, offering documents